1. Field of the Invention
The present invention generally relates to devices for pumping liquids and, more particularly, to positive displacement pumps having reduced fluid-borne noise.
2. Description of the Prior Art
Pumps have long been used in the handling of fluids of both liquid and gaseous phases and many designs have been developed for various applications. For purposes of conveying an appreciation of the present invention, pump designs may be considered to fall generally into two classes. One class of pumps is designed to move a large volume of fluids at a low pressure differential. Most such designs use an impeller, as in a centrifugal pump, or other rotary structure, such as in a fan, to provide a more-or-less continuous flow of fluid. Designs which provide for a continuous flow of fluid have been developed which reduce fluid-borne noise (e.g. principally in the form of small pressure variations in the inlet or outlet fluid flow) to very low levels. However, the pressure such pumps can develop is very limited and many designs cannot be adapted to avoid counterflow against a pressure at the pump outlet which is greater than the pressure which can be developed by the pump.
Pump designs which can produce high pressures of fluid at the outlet of the pump and resist counterflow are generically referred to as positive displacement pumps. The "positive displacement" of fluid by such pump designs in order to achieve high pressures inherently has the effect of quantizing the outlet flow into discrete pulses and the fluid-borne noise at the inlet and outlet of the pump is inherently high. This fluid-borne noise may also be propagated through the pump housing and other structures attached to the pump, particularly at the relatively low frequencies generally associated with positive displacement pumps. This noise is generally insusceptible of reduction without extensive acoustic filtering or isolation treatments which, to date, have been far less than fully effective for many applications. Currently, only about 20 dB of noise reduction can be obtained from acoustic filtering and only about 10 dB of noise reduction can be obtained from isolation for noise characteristic of currently available positive displacement pumps.
Fluid-borne noise is of some concern in virtually all pump applications since the energy required to produce acoustic noise is a component of energy loss and inefficiency. Also, since some pump designs may be extremely large and may be used in applications where personnel are present, noise can prove to be a concern for reasons of safety and health of those personnel. In some applications, however, noise is far more critical. In water-borne vessels, for example, fluid-borne acoustic noise, especially at low frequencies, is only slightly attenuated by water and may be propagated over extremely long distances. In other applications, such as in electro-hydraulic actuators, hydraulic motors, reverse osmosis charging pumps and submersible vessel (manned or unmanned) deballasting, desirable noise levels are more than 50 dB below levels generally produced (without acoustic treatment) by the quietest of positive displacement pumps now available.
Among the quietest current designs for positive displacement pumps currently known is the so-called crescent internal gear (CIG) pump. Gear pumps, in general, form a seal between the meshing teeth of at least two gears and, in other regions of the gears which are not meshed, a close clearance is provided between the tips of the gear teeth and a surface such as the interior of the pump housing, to allow discrete volumes of fluid in the interstices of the unmeshed gear teeth to be positively displaced. The volume of fluid which can be displaced is therefore a function of the volume of the interstices between the unmeshed teeth and the teeth of gear pumps are ordinarily designed to be as large as possible consistent with good meshing of gear teeth and limitation of mechanical noise attributable thereto.
The CIG type of gear pump includes a drive or pinion gear which meshes with the interior of an outer, annular, driven or ring gear. Both gears rotate within a housing but on different parallel axes which are spaced from each other. A substantial portion of the generally crescent-shaped space between the tips of the gear teeth at unmeshed locations is filled with a crescent-shaped insert to provide a close clearance at the tips of the gear teeth. The present state of the art now optimizes CIG pumps by placing 13 teeth on the drive gear and 17 teeth on the annular driven gear. The crescent-shaped insert, likewise, has been optimized to have an angular extent of about 135.degree. with respect to the driven ring gear (and a lesser angle with respect to the pinion drive gear) in order to provide a close clearance with the tips of three or four teeth on the drive gear and five or six teeth on the driven gear and thus alternate the times of opening of the interstices between teeth into the outlet region of the pump.
While the fluid-borne noise from a CIG type of pump is among the lowest of all positive displacement pumps, efforts to further reduce fluid-borne noise such as angularly staggering the gear teeth in respective axial portions of the gears (which increases second harmonic noise), improving gear tooth profiles to deliver a more constant flow rate and determining end locations of the crescent insert such that the tips of gear teeth arrive at the ends of the insert alternatingly, have only achieved slight (e.g. a maximum of about 10 dB) improvement in noise reduction in comparison with the amount of noise reduction which is considered desirable.
Another characteristic of gear pumps in general and CIG pumps in particular is the inherent increase in noise with increases in the pressure differential across the pump. This problem has been particularly intractable for noise reduction efforts undertaken to date and is particularly critical for deballasting pumps in submersible vessels since the pump must produce a pressure greater than that exerted on the submersible vessel by the surrounding water which, of course, increases rapidly with the depth to which the vessel is submerged. Since both the noise level and the nature of the noise (e.g. harmonic content) may change with pressure, acoustic filtering and isolation of the inherent noise of the pump cannot be an effective solution to the problem of fluid-borne pump noise.